The present invention relates to vehicle braking systems and in particular to disc brake systems used in high performance vehicles. In conventional single-piece disc brake systems, the brake rotor is rigidly attached to the wheel or hub of the vehicle. With this type of attachment method, the wheel runout must be controlled to very tight tolerances. Otherwise, the runout of the wheel will be transmitted to the disc brake rotor, which can cause the brakes to chatter during braking. This is undesirable under any circumstances, but potentially dangerous in high performance and racing applications. Additionally, since the typical one-piece disc brake rotor has a bell shaped portion in addition to the disc shaped portion, thermal expansion of the disc portion of the brake rotor during braking introduces severe stresses on the bell portion of the rotor. Cyclic heating of the disc rotor and corresponding residual stresses can lead to brake rotor warpage.
As a result of the deficiencies of conventional one-piece disc brake rotors, two-piece “floating” disc brake assemblies have been employed in many high performance applications. Two-piece floating disc assemblies have several benefits. First, by using an aluminum bell for the hub section, a great deal of weight can be saved. Since the disc brake is both rotating and unsprung, the lighter aluminum bell benefits acceleration, braking and handling of the vehicle. Second, because the disc portion of the brake rotor is detached from the bell portion of the rotor, it can expand and contract with the cyclic heating and cooling inherent in normal braking applications without introducing stresses into the bell portion of the rotor. Therefore, warpage of the disc is virtually eliminated.
Controlling the tolerances between the disc and bell (or hub) portion of a two-piece brake assembly is critical for proper performance. Excessive circumferential clearance between the hub and bell portion can lead to severe impact loads when the brakes are applied. Similarly, axial clearance must be tightly controlled. If too little axial clearance is provided, runout of the bell will be transmitted to the disc producing undesirable chattering, and as the brakes heat up during braking, the disc portion of the two-piece floating brake assembly will bind against the hub introducing undesirable stresses. If too much axial clearance is provided, the disc portion can develop harmonic oscillations leading to poor performance, excessive wear, and even brakage of the disc and/or hub.
A prior art two-piece floating disc brake assembly as shown in FIG. 1 comprises a hub 12 and a disc 14. Hub 12 includes a plurality of axially extending bosses 16 that engage a plurality of radially extending slots 18 formed in disc 14. Disc 14 is retained against hub 12 by means of a plurality of retainers 20 which are secured to hub 12 by a corresponding plurality of threaded fasteners 22 and corresponding nuts 24. Circumferential clearance between disc 14 and hub 12 is maintained by machining slots 18 slightly wider than bosses 16. Axially clearance is maintained by machining disc 14 to be slightly thinner than the height of boss 16.
The prior art two-piece floating disc brake assembly is capable of performing superlatively as long as the tolerance between slots 18 and bosses 16 are maintained precisely. Because the axial clearance, however, is defined by two tolerances (the height of bosses 16 and the thickness of disc 14) elementary statistical tolerancing teaches that the actual clearance may have twice the tolerance of either of the two parts. For example, if the height of bosses 16 are held to a tolerance of ±0.005 inch and the thickness of disc 14 is also held to ±0.005 inch the actual axial clearance can be from a line-to-line fit, which would cause the disc to chatter from the hub runout, up to an axial clearance of 0.020 inches which would allow the disc to oscillate. Moreover, since the axial clearance is determined in part by the thickness of the disc, the axial clearance is difficult to control because the absolute thickness of the rotor is especially difficult to control while at the same time controlling the critical features of flatness and parallelism. Moreover, the clearance changes each time the brake rotor is resurfaced or replaced. Accordingly, what is needed is a two-piece floating disc brake assembly in which the axial clearance is defined by a single part rather than the interaction of multiple parts, so that axial clearance can be more easily controlled. Moreover, what is needed is a floating disc brake assembly in which the axial clearance is not dependent on brake rotor thickness, so that the clearance does not change if the rotor is resurfaced or exchanged.